Mass spectrometry (MS) is a well-known technique for obtaining a molecular weight and structural information on chemical compounds. According to mass spectrometry, molecules may be “weighed” by ionizing the molecules and measuring the response of their trajectories in a vacuum to electric and magnetic fields. Ions are “weighed” according to their mass-to-charge (m/z) values.
In tandem mass spectrometry, precursor ions are selected by the first mass filter. The selected ions are accelerated to a desired kinetic energy, typically by accelerating them across a potential difference into a gas-filled collision cell. Collisions in the presence of the collision gas induce fragmentation, also known as collision induced dissociation (CID). Fragment ions are then filtered by the second means of mass filtering. The product of the collision cell length and the pressure (length×pressure) is known as the target thickness. The incoming beam of precursor ions requires a certain target thickness in order to be fragmented and in order for the fragments to then be thermalized. The type of fragment ion produced, and the number of fragment ions produced, are in part determined by the collision energy, collision partner and pressure of the collision cell.
Generally, a collision cell includes multiple elongated ion guide rods, grouped in two poles, enclosed in a shell or housing. Two opposed electrically conducting electrodes, each forming an electrostatic lens at each end of the collision cell complete the enclosure. Most collision cells include parallel ion guide rods, often arranged in sets of two, three or four rod pairs. RF voltages of opposite phases are applied to opposing pairs of the rods to generate an electric field that contains the ions as they are transported from the entrance to the exit.
Conventionally, ions are accelerated across a potential drop of 20-50V or more, with the pressure maintained between 1 to 10 mTorr by introduction of collision gas (N2, air or Ar). The length of the collision cell is typically not less than 15 cm since the ions must experience a minimum number of collisions at the limited pressure range of 1 to 10 mTorr. Higher pressures and shorter lengths are not possible with conventional cells due to restrictions in pumping technology.
Therefore, conventional pumping systems require that collision cells are long, increasing the size and therefore limiting ease of use and increasing the complexity of mass spectrometers.
As well, because conventional collision cells operate in a limited pressure regime, they produce a restricted set of fragmentation patterns that may not always be useful, particularly for large molecules, greatly limiting the information content of a measurement. This is particularly true for large molecular ions for which low pressure CID is not useful.
Further, due to the length of the collision cell, an additional axial field is often superimposed on the collision cell which is required to move ions along from the entrance to the exit. The need for the axial field is significant as ions tend to slow down almost to a halt without it. A suitably shaped axial field may, for example, be produced by manipulating the shape of the electric field produced by the parallel rods. The relative voltages on the neighboring rods determine the axial field. Unfortunately, ion guides that rely on the shape of the electric field between the rods to produce an axial field tend to distort the electric field asymmetrically, reducing mass range and sensitivity. Other known ion guides use auxiliary electrodes in conjunction with the guide rods to produce a suitably shaped axial electric field. A DC voltage is applied to the auxiliary electrodes that, in conjunction with the rod set, serve to produce an axial field. Unfortunately, the use of auxiliary electrodes tends to be complex and expensive. For example, for 2n guide rods in the ion guide, there will be 2n auxiliary rods, giving a total of 4n rods, increasing cost and complexity substantially.
Accordingly, there remains a need for a collision cell that is small in size, provides an axial field and as well provides for alternative fragmentation pathways not available in currently available collision cells, while optimizing the use of differential pumping technology.
Additionally it is desirable to provide an improved mass collision cell for mass spectrometers which is more compact and economical than presently available collision cells.